Process for production of aromatic polycarboxylic acids



United States Patent PROCESS FOR PRODUCTION OF AROMATIC POLYCARBOXYLIC ACIDS Ewald Katzschmann, Witten (Ruhr), Germany, assignor to Imhausen & Cie., G. m. b. H., Witten (Ruhr), Germany, a corporation of Germany No Drawing. Application July 31, 1952, Serial No. 301,997

Claims priority, application Germany August 6, 1951 6 Claims. (Cl. 260-524) This invention relates to the production of aromatic carboxylic acids from alkylaromatic compounds. More particularly, it relates to the production of the various isomeric phthalic acids from the corresponding xylene isomers. It relates especially to the production of terephthalic acid from p-xylene.

In the application, Serial No. 280,387, filed April 3, 1952 of Ewald Katzschmann, there is described a highly advantageous process for converting p-xylene practically quantitatively into the terephthalic acid or its esters at elevated temperatures solely with the aid of oxygen or oxygen-containing gases. In this process the terephthalic acid-containing p-toluic acid obtained as an intermediate product is esterfied. The terephthalic acid ester, if desired, is removed, and the p-toluic acid ester is oxidized with oxygen or oxygen-containing gases. Surprisingly, it has been found that in the oxidation of p-toluic acid esters with oxygen practically only the nuclear methyl group is attacked while the alcohol radical combined with the carboxyl group of the p-toluic acid remains unattacked.

More particularly, this application discloses that the oxidation of p-toluic acid esters with oxygen or oxygencontaining gases at elevated temperatures in liquid phase, under pressure if desired, and advantageously in the presence of oxidation catalysts, proceeds at a rather fast rate and gives excellent yields of pure terephthalic acid monoesters. Moreover, under conditions of oxidation practically no ester interchange of the terephthalic acid monoester occurs so that the latter is obtained in practically pure form.

The aforesaid application also discloses that the inventive idea thereof is capable of generalization. Thus, in the oxidation of alkylaromatic compounds other than the xylenes, the rate of oxidation to the corresponding aromatic polycarboxylic acid is in no way uniform. In the oxidation of other alkylaromatic compounds intermediate carboxylic acids are formed which, like p-toluic acid in the case of p-xylene, are much harder to oxidize further than the starting material or other intermediates, so that the oxidation to polycarboxylic acids undergoes a marked retardation. As in the oxidation of p-xylene, the rate of oxidation of other alkylaromatic compounds to the corresponding polycarboxylic acids also may be greatly increased if the intermediate carboxylic acids formed are converted into esters, and these are further oxidized.

It has now been found that in the case of p-xylene or any other alkylaromatic compound, the rate of oxidation to the corresponding polycarboxylic acids also may be greatly increased if the intermediate carboxylic acids formed, which are relatively more dilficultly oxidizable than the starting material or other intermediates, are converted into salts, and these are further oxidized. Thus, difticultly further oxidizable carboxylic acids occurring as intermediates, which, therefore, contain besides one or more carboxyl groups also alkyl radicals and/or partially oxidized alkyl radicals, are converted to salts in the known manner and the resulting salts are further oxidized. In the case of p-toluic acid obtained in the oxidation of p-xylene, for example, it is converted to a salt of p-toluic acid and the salt further oxidized to a salt of terephthalic acid. Similarly, m-xylene may be converted to a salt of m-toluic acid and the salt further oxidized to a salt of isophthalic acid. Also, o-xylene may be converted to a salt of o-toluic acid and the salt further-oxidized to a salt of o-phthalic acid.

The difficultly oxidizable intermediate carboxylic acids may be converted into any one of a variety of salts. However, the lead salt is particularly suitable for the purpose of the invention.

The fundamental idea of the present inventionfacilitating the oxidation of alkylaromatic compounds to the corresponding polycarboxylic acids by the conversion of difficultly oxidizable intermediates to more readily oxidizable salts-can be applied not only to the various methylbenzenes, such as xylenes, mesitylene, pseudo cumene, durene, etc., but also to alkylaromatic compounds with longer alkyl chains such as methylethylbenzenes, cymenes, etc., and also to alkylated polynuclear aromatic compounds such as naphthalene, diphenyl, anthracene, etc. v

The conversion of relatively dilficultly oxidizable intermediate products of oxidation into more readily oxidizable salts may, if desired, also be performed repeatedly; thus, alkylmonocarboxylic acids formed on oxidation of polyalkylaromatic compounds may be converted to salts and these products oxidized to alkyldicarboxylic acids, followed by conversion to salts, and the latter further oxidized to aromatic tricarboxylic acids, which may still contain alkyl groups.

The conversion of the sparingly oxidizable intermediate carboxylic acids to more easily oxidizable salts is preferably carried out before their concentration in the reaction mixture has increased to such an extent that the rate of oxygen absorption has appreciably decreased. It is fundamentally without significance whether the conversion of the diflicultly oxidizable intermediate carboxylic acids to the more easily oxidizable salts takes place after their complete or partial separation from the other components of the reaction mixture, or in their presence; the most suitable method in each case can be determined by a simple test.

it is especially advisable to remove the diflicultly oxidizable intermediate carboxylic acids continuously, or at short intervals, from the reaction mixture or from a part thereof, or to convert them therein into more readily oxidizable salts, to make up the consumed starting material and, if necessary, to remove the products of oxidation in such a way that the composition of the reaction mixture and the rate of reaction will remain substantially constant.

The oxidation steps of the process of this invention are carried out under essentially the same conditions as discussed in the aforesaid application, Serial No. 280,387.

By the method according to the invention, polyalkylaromatic compounds can be readily converted into the corresponding polycarboxylic acids.

The following example is given in illustration of the invention.

ICC

Example m-Toluic acid is obtained in the known manner by oxidizing m-xylene with air in the presence of cobalt salts of C4-C9 fatty acids (the forerun acids obtained in the oxidation of paraflin) 1000 g. of lead m-toluate is prepared by reaction of potassium m-toluate with lead acetate in the known manner. The lead m-toluate is then oxidized for 48 hours at 180 C. with 1.5 liters of air per minute. The oxidate is treated at the boiling point with 750 g. of KOH in alcohol, filtered from the lead oxide, the filtrate freed of alcohol, diluted with water, filtered, and acidified with HCl, whereupon 550 g. of acid precipitates. The acid mixture is extracted at the boiling point with 3 liters of benzene, whereupon 30 g. of isophthalic acid remain undissolved; i. e., 5.5%, calculated on the acid mixture isolated from the oxidate.

If 1000 g. of free m-toluic acid is oxidized with 0.2% of the cobalt salts of C4C9 fatty acids with 2 liters of air per minute at C. for 48 hours, only 12 g., or 1.2 of isophthalic acid is obtained with analogous processmg.

Besides the salts of the alkylaromatic acids with lead, also the corresponding salts with thallium, copper, iron, manganese etc. are suitable as the starting material for the process of the present invention. The rate of oxidatiorr may be" increased by' the addition of catalysts; f; i; salts of cobalt.

What I claim and desire to protect by Letters Patent 1. In the preparation of an aromatic pol ycarboxylic' acid by oxidation of'analkyl aromatic compound, wherein there is produced a' relatively-'difiiculty oxidizao'le carboxylic acid as an intermediate product, the process which comprises converting the said intermediate acid' to"the corresponding salt of a metal from the group-com sisting of lead, iron, copper, manganese and' thallium, subjecting the resulting salt to oxidation in liquid phase by an oxygen containing gas under substantially anhydrous-conditions to produce the same salt of thearorna'tic polycarboxylic acid, and converting the resulting salt to the corresponding acid with separation of the by-product metal compound.

2. In the preparatio'n'of aphthalic acid by oxidation of a xylene, wherein there is produced as" an intermediate product a relatively difiiculty oxidizable' toluic acid, the process which comprises converting the toluic acid to a salt of a heavy metal from the group consisting of lead, iron, copper, managnese and thallium, subjecting-the resulting toluate to-oxidation in liquidphase byan oxygen containing gas in a substantiallyanhydrous reaction mixture to produce a phthalic acid salt, convertingthe said salt tothe corresponding phthalic acid, and recovering the phthalic acid substantially free of any by-product compound of the said metal.

3.111 the preparation of terephthalic' acid by oxidization of p-xylene, wherein p-toluic'acid is producedas an intermediate product, the process. which comprises converting the p-toluic acid to a toluate of a metal from the group consisting of lead, iron, copper, manganese'and thallium, oxidizing the toluate in liquid phase. to the metal salt of terephthalic acid by action of an oxygen containing gas in asubsta'ntially anhydrousreaction mix: ture; acidifying to convert: the said tereph'thalic acid to terephthalic' acid,. and recovering the terephth'alic acid substantially free of. any'by-product compound" oflthe' said metal.

4. In the preparation of terephthalic acid by oxidizationof p-xylene, wherein'p-toluic acid is produced as an intermediate product, the process which comprises converting the p-toluic acid to the corresponding lead toluate, oxidizing the said toluate in liquid phase in a substantially anhydrous reaction mixture by action of an oxygen-containing gas to produce the lead salt. of terephthalic acid, acidifying to' convert' the said terephthalic salt to terephthalic acid, and recovering the terephthalic acid substantially free of any by-product lead compound.

5".- In the preparation of phthalic acid' from the corresponding toluic acid, the. process which. comprises converting the toluic acid to the toluate of'a metal from the group consisting of. lead; iron, copper, manganese and thallium, oxidizing the said toluate in liquid phase in a substantially anhydrous reaction mixture by action of an. Oxygen con'tainin'ggas to'- produce the metal saltof the corresponding phthalic acid, acidifying to convert the phthalic salt to phthalic'acid, and recovering the phthalic acid substantially free of any by-product compound of the said metal. V

6. In the preparation of terephthalic' acid' from ptoluic acid, the process which comprises converting the p-toluic acid to. the corresponding lead toluate, oxidizing the said toluate in liquidphase in' a substantially anhydrous reaction mixture by action of an oxygen containing gas to produce the lead salt of terephthalic acid, acidifying to convert the terephthalic' salt to terephthalic' acid, and recovering the terephthalic acid substantially free of any by-productlead compound.

References Citedin the file'of this patent UNITED STATES PATENTS 1,815,985 Pansegrau- July 28', 1931 2,245,528 Loder June 10, 1941 2,479,067 Gresham Aug. 16, 1949 2,552,268 Emerson etal. May 8, 1951 2,559,147 Emerson et a1. July 3, 1951' FOREIGN PATENTS 623,836 Great Britain May 24',- 1949 

1. IN THE PREPARATION OF AN AROMATIC POLYCARBOXYLIC ACID BY OXIDATION OF AN ALKYL AROMATIC COMPOUND. WHEREIN THERE IS PRODUCED A RELATIVELY DIFFICULTY OXIDIZABLE CARBOXYLIC ACID AS AN INTERMEDIATE PRODUCT, THE PROCESS WHICH COMPRISES CONVERTING THE SAID INTERMEDIATE ACID TO THE CORRESPONDING SALT OF A METAL FROM THE GROUP CONSISTING OF LEAD, IRON, COPPER, MANGANESE AND THALLIUM SUBJECTING THE RESULTING SALT TO OXIDATION IN LIQUID PHASE BY AN OXYGEN CONTAINING GAS UNDER SUBSTANTIALLY ANHYDROUS CONDITIONS TO PRODUCE THE SAME SALT OF THE AROMATIC POLYCARBOXYLIC ACID, AND CONVERTING THE RESULTING SALT TO THE CORRESPONDING ACID WITH SEPARATION OF THE BY-PRODUCT METAL COMPOUND. 